Organ transplant tissues created from stem cells have been seen as a medical possibility for decades. An experiment with human liver stem cells points to one promising path to this kind of medicine.

An international stem cell research team reports Wednesday that they have grown functioning human liver tissues in mice.

The human liver “buds” implanted in the mice represent a first experimental step in growing replacement organs from stem cells for transplants, such as liver, pancreas and kidneys, says the research team headed by Japan's Takanori Takebe of the Yokohama City University Graduate School of Medicine. The team relied on a “cocktail” of so-called induced stem cells grown to resemble the nascent liver bud cells used in the experiment…

…Discovered in 2006, induced stem cells are grown from mature tissues, typically skin cells, into the unspecialized stem cell state that allows for their cultivation into a wide variety of cell types, from brain to blood to liver cells. In the new study reported in the journal Nature, the Japanese team reports they essentially mixed together a trio of induced stem cells that included liver cells to see what would happen.

“They unexpectedly self-organize to form a three-dimensional liver bud — this is a rudimentary liver,” Takebe says. Each bud was less than a fifth of an inch across in size and consisted of about 180,000 cells. One novelty of the team's approach was in mixing the cell types to re-create the environment of the early liver, where more often stem cell researchers very carefully segregate cell types from each other in order to grow pure cell colonies of a desired variety.

Implanted into mice, the liver buds released human liver enzymes much more effectively than more massive amounts of liver precursor cells implanted alone in mice. The buds also developed blood vessels and grew to resemble normal liver tissues within about two days of implantation. As a final test, the researchers induced a kind of chemically induced liver failure that resembles the disease in people in 12 of the mice, and they report that implanted liver buds helped the mice survive.